Yup, Queue is right. I just did some testing, it looks like if you try to use characters that don't exist in the game (like parenthesis or brackets), the string writing function just won't write anything, and if you try to write anything above the ASCII 0x7F max value (which is how it stores the data), it'll just write a 'B'.

EDIT: Uh, whoops. I posted a whole tutorial, but it turns out that I had the entirely wrong font image set referenced. So yeah, I'm going to have to look into this further. As of right now, you can't insert new characters into the Mario Kart 64 ROM.